The man is in town this week, and on the Nokia message board they're asking, "Does anyone know how to get Stan as a ringer tone?" Which is all very postmodern and interesting, since the hit song overtly references the brave new world of telecommunications: "Dear Slim, I wrote you but you still ain't callin' / I left my cell, my pager, and my home phone at the bottom . . ."

The variety of possible ways in which the singer can get in touch with his doomed fan are appropriately overspecified, considering Slim (Eminem's persona) at first doesn't reply to his fan's obsessive letter. Eventually he does get round to writing back: "Dear Stan, I meant to write you sooner, but I've just been busy . . ." He continues: "I'm sorry I didn't see you at the show, I must have missed you. / Don't think I did that shit intentionally just to diss you."

The irony in these lines is delicious, but just how good are Eminem's lyrics? Is all the fuss about him justified, or is it a case of hype over substance? In fact, a brief examination of Stan reveals it to have all the depth and texture of the greatest examples of English verse. To use the singer's own language, it's as "fat" as Robert Browning - and it is with the Victorian master of sly irony that Eminem's true "underground" work is done, just as much as with Scam and Ruckus, the bands noticed in the song.

Of course, it's nothing new to make great claims for song lyrics. The work listed in The Poetry of Rock'n'Roll, a 1970s anthology, might seem banal (Donovan?), but it has long been the habit of the more flamboyant Cambridge English dons to put popular music under the full glare of hermeneutic inquiry. It was Christopher Ricks who started it off, writing about Dylan from the 1970s onwards. This Sunday, Ricks gives a talk on the subject on Radio 3. In Bob Dylan Among the Poets, he attacks the tired Keats v Dylan argument that his original pieces gave rise to. It was David Hare who set up the opposition, one that Ricks (quoting the recent Dylan song Not Dark Yet alongside Keats's Ode to a Nightingale) says is actually a unity.

Other dons have followed suit: Eric Griffiths juxtaposing Talking Heads with William Empson in Cambridge lectures during the late 1980s. But it is rare for a singer to combine public outrage and textual richness in quite the way Eminem does.

But who is he, really? Like the Portuguese poet Fernando Pessoa, with his quiverful of pseudonyms, like the coy Eliot of Prufrock, or Walt Whitman's Song of Myself - "Do I contradict myself? Very well then I contradict myself, (I am large, I contain multitudes)", Eminem is a multiple, elusive experience, one that folds about itself like his near-palindromic name (from Marshall Mathers: M 'n' M).

Who the "real Slim Shady" is, in other words, is hard to say. As TS Eliot put it: "The Naming of Cats is a difficult matter, / It isn't just one of your holiday games; / You may think I'm as mad as a hatter / When I tell you a cat must have THREE DIFFERENT NAMES."

Like Macavity, Eminem always has an alibi, and one or two to spare. It is as if there were always someone else walking beside him. The mistake his critics make is to see the songs as direct statements by the singer rather than discrete aesthetic objects. Is it inconceivable that the man who wrote Stan should also want to stab you in the head "if you're a fag or a les"? Not necessarily, but Eminem's critics should "relax a little" as he advises Stan - should consider that this might be an artist toying with the place where celebrity and palatability meet, by passing deliberately inflammatory statements and by parodying less thoughtful rappers.

The "I" voice has long been subject to modulation in English and American poetry. Browning's great dramatic monologues, such as My Last Duchess or Porphyria's Lover, are the classic examples of this. Other poets, including Tennyson, Hardy, Kipling, Frost, Pound and Eliot himself, also mastered the form.

According to the Handbook to Literature, a dramatic monologue is a poem "that reveals a 'soul in action' through the speech of one character in a dramatic situation. The character is speaking to an identifiable but silent listener at a dramatic moment in the speaker's life. The circumstances surrounding the conversation, one side of which we 'hear' . . . are made clear by implication, and an insight into the character of the speaker may result."

In My Last Duchess, the speaker is an Italian duke who has had his flirtatious wife murdered, and is showing her portrait to (probably) an envoy from his next father-in-law. A "picture on the wall", as in the chorus to Stan, furnishes the occasion for the poem.

Where Stan differs, and is in some respects more sophisticated - although it is a sophistication only possible in a mass-media, celebrity-driven world - is that both addressee and addresser speak in it. There is a further sophistication in so far as all listeners are in some sense co-opted into the Stan role.

By ironically dramatising two sets of letters, Stan also fits snugly into the tradition of the verse epistle out of which the dramatic monologue developed. It shares, too, some qualities of unreliable narration with Porphyria's Lover, in which another murderer speaks. As with Stan ("the morning rain clouds up my window and I can't see at all"), the weather fits the lunatic's mood: "The rain set in early tonight, / The sullen wind was soon awake, / It tore the elm-tops down for spite."

The speaker in Porphyria's Lover goes on to reveal how much Porphyria loved him - so much that to enable them to be together "her hair / In one long yellow string I wound / Three times her little throat around, / And strangled her. No pain felt she . . ."

If we don't accuse Browning of misogyny and incitement to violence here, we should extend Eminem the same courtesy. We shouldn't fall into the trap of Stan himself, who tells Slim: "See, everything you say is real, and I respect you 'cause you tell it."

It's clever that "see" - as if you might hear, behind the layers of impersonation, an authentic voice saying "Can't you see ? This is a story, stupid". It also has a touch of the bardic "lo!": the poet saying hey, look what I have made.

Yet neither that voice nor even the one telling Stan that he's "got some issues" (you bet) and that he needs counselling and that he and his girlfriend need each other, can be said to be "the true voice of Marshall Mathers" any more than the incitements to sexual violence can. The younger fans who buy Eminem's albums probably understand this instinctively. Just as today's cinema-goers are more adept with movie conventions than their parents, so Eminem's youthful fans can see between the lines, and the publicity stunts, that the tabloids lap up so eagerly. The joke is on the editors, in fact.

But what about those kids who do take it for real? As Mathers himself put it in an interview (sounding more like Paul Johnson than Public Enemy), "There are kids out there who, believe it or not, want to be the have-nots." The question of how much sympathy or disdain Mathers has for them, for the Stans of the world, adds another dimension to the song.

As does the question of how much he can be said to encourage them. To what extent, actually, can artists be held responsible for their works? This is a question poets have asked themselves for centuries, especially when dealing with tragic material. "Go litel bok, go, litel myn tragedye", said Chaucer, sending Troilus and Criseyde out into the world like a paper boat. He knew it wasn't as simple as that - though it isn't just a question of words being akin to deeds either. "Did that play of mine send out / Certain men the English shot?", Yeats asks, looking back on the Easter Rising in The Man and the Echo (Last Poems: 1936-39). Eminem's talent lies in his turning of these issues into the material of both his work and his personae. "You could have rescued me from drowning," Stan writes; "Why are you so mad?" Slim replies, "Try to understand . . ."

Try indeed. Stan, and much else in Eminem's oeuvre, explores humanity's most profound experience: not just madness, but also terror, melancholy and (not least) laughter. In this view Eminem is neither the "authentic voice of disaffected working class youth" (Independent on Sunday), nor "a nasty little yob"(ditto), but a rapper whose genius is, principally, poetic.

You can hear that genius in the disposition of poetic stress in that opening verse of Stan - "my cell, my pager and my home phone". The pick-up of metre and sense between "cell" and "phone" puts a lot of weight on "cell" - making us think, perhaps, of the other type of cell might be in order for Stan.

And of the phone tones themselves of course. Oh, and while we're on the subject, for the Nokia 3210 you can get the Stan ringer tone at www.dialaring.com.

 Giles Foden is the Guardian's deputy literary editor. His most recent book, Ladysmith, is published by Faber and Faber at £6.99.